Horticulture and Agriculture

Last year during the Summer of Weeds I inadvertently wrote about several edible weeds, one of which I even ate. It’s not surprising that so many weeds are edible; there are plenty of plants out there – both native and introduced – that are, despite the fact that most of us stick with whatever is made available at the grocery store. Some edible weeds, dandelion included, were once commonly grown for food, while other weeds are close relatives of present day agricultural crops. The more I read about these things and the more my weeds obsession grows, the more I feel compelled to eat them (the edible ones, at least). Hence, a new series of posts: Eating Weeds.

I might as well start with an easy one. Chenopodium album, or lambsquarters, which I wrote about last summer, is a close relative of a number of common crops and a spitting image of quinoa. It happily grows alongside other plants in our vegetable gardens without even being asked to. It is highly nutritious and palatable – particularly the young leaves – and can be eaten raw or cooked. It is often compared to spinach and can be prepared and used in similar ways.

lambsquarters seedling (Chenopodium album)

For the purposes of this post, I decided to try lambsquarters pesto. While pesto is traditionally made using basil leaves, all kinds of other leaves – or combinations thereof – can be substituted. I have made pesto with parsley, which was interesting, as well as watercress, which was delicious. The possibilities are endless. So, why not lambsquarters?

Making pesto is incredibly simple. Blend together a combination of leaves, garlic, nuts or seeds, Parmesan cheese (or something similar), olive oil, salt, and pepper. Pine nuts are traditionally used to make pesto, but like the leaf component, a number of different nuts or seeds can be substituted. I rarely make pesto with pine nuts because, despite being delicious, they are pricey.

lambsquarters pesto

I made two batches of lambsquarters pesto. For the first I used walnuts, and for the second I used sunflower seeds. Both batches were delicious. How could they not be with all of that garlic and cheese in there? Lambsquarters is not a very bitter or strong-tasting green, so lambsquarters pesto might be perfect for anyone who is otherwise not fond of pesto (although that is a stance that I personally cannot fathom).

This is definitely something I will make again. I understand the frustration people have with lambsquarters. It can be prolific and hard to eliminate from a garden. Luckily, it makes an excellent pesto.

Resources:

This series of posts was inspired in part by the book Dandelion Hunter, in which the author, Rebecca Lerner, attempts to go a full week eating only things she is able to forage in her hometown of Portland, Oregon. As you might imagine, many of the plants she forages are weeds.

Is there a plant family that consists of more weedy species than the mustard family? Asteraceae and Poaceae, for sure. Fabaceae or Lamiaceae, perhaps. Regardless, Brassicaceae is replete with dozens of species – mostly annual – that are skilled at taking advantage of the disturbed environments that humans are in the habit of creating.

It helps that the mustard family is so large: 372 genera and over 4,000 species distributed across the globe. Around 55 genera are said to occur in North America. Most of the plants in this family are herbaceous; few are shrubs. Foliage is aromatic, especially when crushed. Flowers are particularly distinctive. Each flower has four petals – in some species petals are divided, giving the impression that there are more than four – arranged in the shape of a cross or “X.” Flowers are often small, have 4 tall stamens and 2 short stamens, and commonly come in white, yellow, pink, or purple. They are arranged on a raceme, which is typically either tall and straight or compact and flat-topped.

Fruits in the mustard family are capsules with two compartments separated by a clear membrane. The capsules may be at least three times longer than they are wide, in which case they are referred to as a silique; or they may be less than three times longer than they are wide and referred to as a silicle. This is a curious distinction, and it doesn’t tell you all that much. It’s more important to understand that the capsules of mustards can come in various sizes and shapes, and that some can be long and narrow while others are short and either round or angular.

Despite the size or shape of the capsule, enclosed are numerous seeds – sometimes dozens. Surely one of the reasons why plants in the mustard family are so successful at proliferating is their ability to produce thousands, even tens of thousands, of seeds per plant. The seeds are typically tiny; and while they may not make it very far from the parent plant, they are numerous. Depending on the species, they can also remain viable for years, affording them the opportunity to sprout whenever conditions are right. You may have heard the biblical verse about faith the size of a mustard seed giving one the ability to move mountains. Size seems irrelevant here, so how about faith as tough, resilient, opportunistic, and resourceful as a mustard seed? If a mountain can be moved, mustards might be the one to do it.

While it isn’t the scope of this post, it’s worth mentioning the chemical compounds present in mustards that give them the flavors and health benefits we enjoy as well as the toxicity that can harm us and any other organisms that dare consume them. Glucosinolates, which are present in various concentrations depending on the species, are a defining characteristic of plants in the mustard family. They contribute to the spicy-ness of things like horseradish, radish, and condiment mustard while also acting as a natural insecticide, deterring herbivory.

And now on to the cast of characters:

Whitetop (Lepidium spp.)

Garlic mustard (Alliaria petiolata) – a noxious weed in many parts of North America – is fortunately not an issue in southwestern Idaho, otherwise it would be first on the list. Instead, we deal with whitetop – a noxious weed in Idaho and several other states. As the common name suggests, individual plants – up to two feet tall – are topped with a dense cluster of tiny, white flowers. Seed production in this group isn’t as abundant as other mustards; instead, the tour de force are their rhizomes. Whitetop is a perennial plant that spreads aggressively via underground stems as well as root fragments and can easily form expansive, dense patches, outcompeting other plants in the area.

Another common name for this group is hoary cress on account of their gray-green, fuzzy foliage. They are further distinguished by the shape of their seed pods: lens-podded hoary cress (L. chalepense), heart-podded hoary cress (L. draba), and globe-podded hoary cress (L. applelianum).

white top (Lepidium sp.)

white top (Lepidium sp.)

Tansymustard (Descurainia spp.)

There are two species of tansymustard (also known as flixweed) that occur in my part of the world, one is native and the other is introduced from Europe. They are indistinguishable to my untrained eye. If I have seen them side by side, I wouldn’t have known it. They are both annuals and can be as short as a few inches to over two feet tall. They have highly dissected, fern-like leaves and tiny, pale yellow or green-yellow flowers. The seed pods are very skinny and around an inch long. Each pod can hold 40 seeds, and a large plant can produce over 75,000 seeds. They are quick to take advantage of disturbed soil and come up in abundance after a fire. I’m not sure what it is about this year, but they have been particularly prolific this spring.

tansymustard (Descurainia sp.)

tansymustard (Descurainia sp.)

Blue Mustard (Chorispora tenella)

Also known as musk mustard or crossflower, this sticky, stinky, annual plant apparently makes cow’s milk taste funny; however some people still enjoy eating it. It can get to about a foot and a half tall, and is adorned with pretty, little, blue-purple flowers. The pointy seed pods split crosswise rather than lengthwise, an uncommon trait in mustards.

blue mustard (Chorispora tenella)

blue mustard (Chorispora tenella)

Desert Madwort (Alyssum desertorum)

Like tansymustard, this species is very similar in appearance to another closely related species, Alyssum alyssoides (commonly known as pale madwort or yellow alyssum). Both are annuals under a foot tall, covered in tiny hairs, with minuscule yellow flowers, and numerous round seed pods. They are adapted to dry, neglected sites.

yellow alyssum (Alyssum dessertorum)

Annual Honesty (Lunaria annua)

If you don’t recognize this plant when it’s flowering, you will when its seed pods ripen. They are thin, round discs up to three inches across. Eventually, the outer layers of the seed pods fall away, and translucent membranes remain, sometimes with seeds still attached. This trait has earned this species common names like money plant and silver dollar. The plants are attractive, reach up to three feet tall, and produce showy, purple flowers, which explains why they are popular ornamentals. However, like other mustards, the proficiency with which they reproduce in abundance via seeds, means they also easily migrate into natural areas and neglected sites.

annual honesty (Lunaria annua)

Hairy Bittercress (Cardamine hirsuta)

This little, quick-growing, fast-spreading annual is a common nuisance in greenhouses and nurseries. On stalks above compact rosettes are borne clusters of white flowers that, as Ken Thompson writes in The Book of Weeds, “are so tiny they are almost invisible.” The slender seed pods burst open at maturity, sending minuscule seeds flying. Brush your hand over a patch of mature hairy bittercress and you will be bombarded.

hairy bittercress (Cardamine hirsuta)

And the list goes on…

I’ve observed several other weedy species in this family recently, but to keep the length of this post reasonable I will just list them here: shepherd’s purse (Capsella bursa-pastoris), clasping pepperweed (Lepidium perfoliatum), spring draba (Draba verna), tumble mustard (Sisymbrium altissimum), and pennycress (Thlaspi arvense). This list only scracthes the surface; there are many other weeds in the mustard family. All deserve to be mentioned, so perhaps another time.

Most of us know creeping charlie as the all-too-often irritating weed which takes over our grassy lawns. This evergreen plant’s life cycle is year round. The garden-invading variety which sprouts little bluish-purple flowers has been given the title Glechoma hederacea (or sometimes Nepeta glechoma) via binomial nomenclature and is a member of the mint family, Lamiaceae. Additional common names for this creeping charlie include ground ivy, catsfoot, and field balm.

Travelers from Europe took the plant with them, distributing it throughout other parts of the globe, and it is now deemed an aggressive, invasive weed in various areas in North America. It has crenate leaves, and its size varies depending on its living conditions. It has two methods of reproduction. The first is made possible by offshoots called stolons (or runners), stems with the special function of generating roots and transforming into more plants. Thus, you will often find not an individual creeping charlie plant, but a whole patch, all of them connected via the runners. The other self-distribution method is simple: seeds.

creeping charlie (Glechoma hederacea) via John Tuttle

The creeper is edible, and if you were in a spot where you didn’t know when your next meal would be, this type of creeping charlie would probably be a welcome source of energy. Wild food educator, Karen Stephenson, suggests its use in simple dishes such as soups and omelets, but that’s probably for those who are cooking at home and not trying to fend for their lives in some forest. Starving in the woods is a bit of an extreme, but it has happened. Glechoma hederacea has also been used for making tea. It contains minerals like copper and iron, as well as a significant amount of vitamin C.

The weed also has a number of possible health benefits such as being a diuretic, anti-inflammatory, and antiviral. However, other researchers have cautioned people to be leery of consuming it as it has been known to be fatal to equines and bovines. It contains chemicals that can discomfit the gastrointestinal tract. It is further suggested that during pregnancy women should not intake any amount of any type of creeping charlie.

Up to this point you may have found the terms I’ve used, such as “this type of creeping charlie,” to be a little odd. In fact, the term creeping charlie does not refer to only a single species of creeper. It’s actually used for several.

Another plant hailed as “creeping charlie” is Pileanummulariifolia of the family Urticaceae, a grouping otherwise known as the nettles. Pilea is the name of the genus of creeping plants; the artillery plant is also classified under this genus. Pilea nummularifolia is also known as Swedish ivy and is often grown as a houseplant. It is native to the West Indies and parts of South America. This viney plant flourishes when supplied with an ample amount of water.

Yet another plant commonly referred to as creeping charlie is Micromeria brownei, synonymously referred to as Clinopodium brownei. It is also used in some teas, but as mentioned above, pregnant women in particular should steer away from consuming it. Apart from the term creeping charlie, a few more common names for this plant include Browne’s savory and mint charlie. Like Glechoma hederacea, Browne’s savory is considered a mint. It produces flowers that are white with hints of purple on the petals and in the throat. This species is quite common in the state of Florida and in parts of Central America; although plants in this genus grow around the world.

Like Pilea nummularifolia, this species loves a good source of water. Its thirst for moisture is so strong, that it can actually adapt itself to an aquatic lifestyle, that is, one which occurs in water and not in dry soil. Many aquarists, people who enjoy keeping aquatic life, love this plant. It can also be trimmed with practically no damage to the plant. It is extremely durable and quite capable of adapting to different circumstances. For instance, Micromeria browneican be situated midground inside a fish tank. The creeping charlie is perfectly at home totally submerged under water. If a plant floats to the surface then it should typically produce flowers. This adds a new dimension to some of the generic aquatic flora which is often used in many tank displays.

There you have it. Three different types of plants that have different uses and dangers, and they are all called creeping charlie. Be advised when you’re talking about true creeping charlie – Glechoma hederacea: the invasive weed with the purple flower – that you remember to specify, because “creeping charlie” could mean one plant to you and some plant from an entirely different family to another.

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John Tuttle is a Catholic guy with a passion for the media and creativity. Everything about science and health interests him. He’s a writer for publications such as ZME Science and Towards Data Science. John has started his own blog as well called Of Intellect and Interest. He’s also a published ebook author and the 1st place winner of the youth category of the 2017 Skeena Wild Film Fest. You can follow him on Facebook here, and he can be reached anytime at jptuttleb9@gmail.com.

Earlier this month, I went to Garden Grove, California to attend the 71st annual meeting of the Western Society of Weed Science. My trip was funded by an Education and Enrichment Award presented by the Pahove Chapterof the Idaho Native Plant Society. It was a great opportunity for a weeds-obsessed plant geek like myself to hang out with a bunch of weed scientists and learn about their latest research. What follows are a few highlights and takeaways from the meeting.

General Session

Apart from opening remarks and news/business-y stuff, the general session featured two invited speakers: soil ecologist Lydia Jennings and historian David Marley. Lydia’s talk was titled “Land Acknowledgement and Indigenous Knowledge in Science.” She started by sharing a website called Native Land, which features an image of the Earth overlayed with known “borders” of indigenous territories. By entering your address, you can see a list of the tribes that historically used the land you now inhabit. It is important for us to consider the history of the land we currently live and work on. Lydia then compared aspects of western science and indigenous science, pointing out ways they differ as well as ways they can be used in tandem. By collaborating with tribal nations, weed scientists can benefit from traditional ecological knowledge. Such knowledge, which has historically gone largely unrecognized in the scientific community, should receive more attention and acknowledgement.

David Marley was the comic relief. Well-versed in the history of Disneyland, he humorously presented a series of stories involving its creation. Little of what he had to say related to weed science, which he openly admitted along the way; however, one weeds related story stood out. Due to a lack of funds, the early years of Tomorrowland featured few landscape plants. To make up for that, Walt Disney had signs with fake Latin names created for some of the weeds.

Weeds of Range and Natural Areas

I spent the last half of the first day in the “Weeds of Range and Natural Areas” session where I learned about herbicide ballistic technology (i.e. killing plants from a helicopter with a paintball gun loaded with herbicide). This is one of the ways that Miconia calvescens invasions in Hawaii are being addressed. I also learned about research involving plant debris left over after logging. When heavy amounts of debris are left in place, scotch broom (Cytisus scoparius) infestations are thwarted. There was also a talk about controlling escaped garden loosestrife (Lysimachia punctata) populations in the Seattle area, as well as a few talks about efforts to control annual grasses like cheatgrass (Bromus tectorum) in sagebrush steppes. Clearly there are lots of weed issues in natural areas, as that only covers about half the talks.

Basic Biology and Ecology

On the morning of the second day, the “Basic Biology and Ecology” session held a discussion about weeds and climate change. As climate changes, weeds will adapt and find new locations to invade. Perhaps some weeds won’t be as problematic in certain areas, but other species are sure to take their place. Understanding the changes that are afoot and the ways that weeds will respond to them is paramount to successful weed management. This means documenting the traits of every weed species, including variations between and among populations of each species, so that predictions can be made about their behavior. It also means anticipating new weed species and determining ways in which weeds might exploit new conditions.

No doubt there is much to learn in order to adequately manage weeds in a changing climate. An idea brought up during the discussion that I was particularly intrigued by was using citizen scientists to help gather data about weeds. Similar to other organizations that collect phenological data from the public on a variety of species, a website could be set up for citizen scientists to report information about weeds in their area, perhaps something like this project in New Zealand. Of course, there are already a series of apps available in North America for citizen scientists to report invasive species sightings, so it seems this is already happening to some degree.

Teaching and Technology Transfer

A highlight of the afternoon’s “Teaching and Technology Transfer” session was learning about the Wyoming Restoration Challenge hosted by University of Wyoming Extension. This was a three year long contest in which thirteen teams were given a quarter-acre plot dominated by cheatgrass with the challenge to restore the plant community to a more productive and diverse state. Each team developed and carried out their own strategy and in the end were judged on a series of criteria including cheatgrass and other weed control, plant diversity, forage production, education and outreach, and scalability. Preliminary results can be seen here; read more about the challenge here and here.

And so much more…

Because multiple sessions were held simultaneously, I was unable to attend every talk. I also had to leave early on the third day, so I missed those talks as well. However, I did get a chance to sit in on a discussion about an increasingly troubling topic, herbicide-resistant weeds, which included a summary of regional listening sessionsthat have been taking place in order to bring more attention to the subject and establish a dialog with those most affected by it.

Ecosystems are everywhere – interconnected and interdependent systems of biology, climate, ecology, and geography. The inside of your house is an ecosystem with its own micro-climate, life (including but not limited to you), and topography. Everywhere you go, you’re in some kind of ecosystem.

The same is more obviously true about your landscape. In my area of the U.S. (southeast Michigan), forests and wetlands are often removed to build suburbs. Both the appropriate soil and ecologically relevant plants are removed from the site. After construction, these areas are re-planted with genetically inadequate plants in poor soil. The ecosystem is modified at a rate faster than most organisms can adapt. Landscape designs common in the suburbs are inadequate in maintaining biodiversity and healthy, natural ecosystems.

In some lucky areas, there are communities doing their best to maintain a strong and natural forest canopy. Leaving secondary forests relatively untouched during construction should be the standard when developing areas for humans.

Ecosystems evolve and change, and one can argue that human-caused mass deforestation is simply another driver of ecosystem evolution. While this may be true, it is a driver that influences the ecosystem at a much greater magnitude than other factors. It just so happens to be mitigable or avoidable altogether.

What can cause an ecosystem to change?

Let’s use the trees in a natural forest ecosystem as an example. Disturbances in any ecosystem drive biological adaptation and behavioral changes in the organisms within it. Disturbances such as fire, wind events, floods, drought, and pathogens alter the forest canopy. Fire may kill smaller trees and wind events can blow trees over. Such disturbances open the canopy and allow dormant seeds to germinate in the new sunlight, which gives additional genetic material a shot in the world.

Ecological disturbance is vital to plants, animals, and microbes because it keeps their genetic material up-to-date with evolving pathogens and changing environments. Up-to-date trees need less work. They are more prepared for their environment and its diseases, as evidenced by their parents successfully reproducing.

We can’t control all ecological disturbances, but in the urban environment we do our best to avoid major ones. Understandably, right? We aren’t fond of wildfire, nor do we want flooding anywhere near our homes.

Applied ecosystem principles on the job

Oftentimes in large, human constructed landscapes, only upper and middle canopies exist; sub-canopy layers are missing. This is surprisingly common in forest ecosystems, especially in suburban areas. Forests like this are considered to have a closed canopy.

Closed-canopy forests are naturally occurring and are not necessarily bad. The thick shade cast by the upper canopy is very dense and prevents most understory growth. Over time closed-canopy forests will evolve and change – large trees or limbs come down in the wind, flooding occurs, lightning strikes, or diseases are introduced. Whatever the disturbance, the newly opened canopy once again helps move the ecosystem forward.

Disturbance by pruning

A client of ours lives on a beautiful property in a dry-mesic southern forest (a closed-canopy forest). Due to all the trees on the property, this client sought advice from arborists. The client’s smart choice lead us to an important solution.

Various large species of both white and red oaks dominate the overstory and upper emergent layers of the canopy. The trunks of these towering trees are far apart. Below these titan trees are some slightly shorter oaks, an american beech, and a few hickory species residing in the midstory. About 40 feet below are various types of moss, some stunted sedges, violets, forest grasses – a sparse herbaceous understory. Beyond that there were several patient serviceberries here and there, and a single red maple, about 1.5 inches in diameter and 15 feet tall at most.

The area has been undisturbed for a long time (it doesn’t even get mowed), and with the presence of oak wilt in southeast Michigan, we steered away from planting anywhere in the root zone, as it poses a risk for oak wilt infection. Sure, we could plant an over-designed landscape to be manicured, but we had other ideas in mind.

Direct application with two solutions

We asked the client how long ago the red maple and serviceberries volunteered themselves into their landscape. Together we traced the germination back to a wind event that knocked a large limb down years ago. The red maple and serviceberries popped up as a result of new sunlight, yet according to the client, these plants hadn’t grown much in height during the last decade or so. Why might this be? A mature plant can close holes in the canopy faster than lower story plants can, so they no longer receive as much light as they once had.

The next time a limb falls, the maple and serviceberries will have another explosive growth spurt. There are also other dormant seeds to germinate every time a disturbance like that occurs. This is an example of another natural phenomenon called forest succession. It is another way forest ecosystems change.

Planting foreign species in place of the native ones takes away important food sources and habitat for surrounding wildlife. So rather than planting cultivar clones and ecologically useless plants – plants that don’t support other lifeforms – into the existing ecosystem, we proposed we could either do strategic crown thinning or just wait for mother nature to do it for them.

Course of action

My associates and I operate on a “less is more” approach. Not touching this ecosystem is our alternative to modifying the canopy. Like a human patient undergoing surgery, cutting open any organism exposes it to infection. In time, either a natural disturbance will come through to modify the canopy, or the trees will naturally shed lower limbs on their own – a process called cladoptosis.

Strategic branch removal will open up the canopy, allowing more sunlight to the ground below, while keeping the trees looking true to their natural form. The climbing team would be using a type of pruning called refracturing. The openings will simulate a wind event disturbance. As a result, the plants that germinate will be the most competitive, hardy, resistant, and genetically up-to-date plants. This truly is “right plant, right place,” provided no invasive buckthorns pop up.

If the customer does want to go forward with disturbance-by-pruning, the proposal is to open the canopy during winter, as most of the canopy are oak trees. The risk of infecting these trees is reduced significantly by pruning in the winter when the vectors for oak wilt are dormant.

The canopy holes would be placed where the homeowner wants more trees. One benefit of pruning the trees is that disturbance is controlled, rather than a wind disturbance causing a chaotic breakage into the house, for example.

Observation would begin early the following spring. We will watch for germination; it’s expected that the plants that do germinate won’t survive the competition.

What’s important about any of this?

The arborist-homeowner relationship highlighted above is an exemplar of proper arboriculture. We offered expertise along with our services. The exchange saved the homeowner hundreds of upfront costs from the installation of a landscape, as well as future maintenance costs.

Assuming it isn’t under human-induced stress, no forest needs human intervention. In this project, we would want to see natural phenomena form the landscape in this client’s yard. It is our preference to leave the current closed-canopy forest alone.

The benefits of using naturally occurring trees are plentiful. In general, up-to-date trees are more prepared for your ecosystem and support the wildlife that co-evolved with them. An ever-increasingly displaced wildlife population will happily occupy new habitat; they’re here too, after all.

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Jeremiah Sandler lives in southeast Michigan, has a degree in horticultural sciences, and is an ISA certified arborist. Follow him on Instagram: @jeremiahsandler

The lettuce we eat is a close relative to the lettuce we weed out of our gardens. Last week we discussed the potential that wild relatives may have for improving cultivated lettuce. But if wild lettuce can be crossed with cultivated lettuce to create new cultivars, can cultivated lettuce cross with wild lettuce to make it more weedy?

Because so many of our crops are closely related to some of the weeds found along with them or the plants growing in nearby natural areas, the creation of crop-wild hybrids has long been a concern. This concern is heightened in the age of transgenic crops (also known as GMOs), for fear that hybrids between weeds and such crops could create super weeds – fast spreading or highly adapted weeds resistant to traditional control methods such as certain herbicides. To reduce this risk, extensive research is necessary before such crops are released for commercial use.

flowers of prickly lettuce (Lactuca serriola)

There are no commercially available, genetically modified varieties of cultivated lettuce, so this is not a concern when it comes to crop-wild hybrids; however, due to how prevalent weedy species like prickly lettuce (Lactuca serriola) are, hybridization with cultivated lettuce is still a concern. So, it is important to understand what the consequences might be when hybridization occurs.

In a paper published in Journal of Applied Ecology in 2005, Hooftman et al. examined a group of second-generation hybrids (L. sativa x L. serriola), and found that the hybrids behaved and appeared very similarly to non-hybrid prickly lettuce. They also found that the seeds produced by the hybrids had a significantly higher germination rate than non-hybrid plants. This is an example of hybrid vigor. Thus, “if hybridization does occur, this could lead to better performing and thus potentially more invasive (hybrid) genotypes.” However, the authors cautioned that “better performing genotypes do not automatically result in higher invasiveness,” and that much depends on the conditions they are found in, the level of human disturbance, etc.

Another thing to consider is that hybrids are not stable. In an article published in Nature Reviews Geneticsin 2003, Stewart et al. adress the “misunderstanding that can arise through the confusion of hybridization and … introgression.” It is wrong to assume that hybrids between crops and wild relatives will automatically lead to super weeds. For this to occur, repeated crosses with parental lines (also known as backcrossing) must occur, and “backcross generations to the wild relative must progress to the point at which the transgene [or other gene(s) in question] is incorporated into the genome of the wild relative.” That is what is meant by “introgression.” This may happen quickly or over many generations or it may never happen at all. Each case is different.

prickly leaf of prickly lettuce (Lactuca serriola)

In a paper published in Journal of Applied Ecology in 2007, Hooftman et al. observe the breakdown of crop-wild lettuce hybrids. They note that “fitness surplus through [hybrid vigor] will often be reduced over few generations,” which is what was seen in the hybrids they observed. One possible reason why this occurs is that lettuce is predominantly a self-crossing species; outcrossing is rare, occurring 1 – 5% of the time thanks to pollinating insects. But that doesn’t mean that a stable, aggressive genotype could never develop. Again, much depends on environmental conditions, as well as rates of outcrossing and other factors relating to population dynamics.

A significant expansion of prickly lettuce across parts of Europe led some to hypothesize that crop-wild hybrids were partly to blame. In a paper published in Molecular Ecologyin 2012 Uwimana et al. ran population genetic analyses on extensive data sets to determine the role that hybridization had in the expansion. They concluded that, at a level of only 7% in wild habitats, crop-wild hybrids were not having a significant impact. They observed greater fitness in the hybrids, as has been observed in other studies (including the one above), but they acknowledged the instability of hybrids, especially in self-pollinating annuals like lettuce.

seed head of prickly lettuce (Lactuca serriola)

It is more likely that the expansion of prickly lettuce in Europe is due to “the expansion of favorable habitat as a result of climate warming and anthropogenic habitat disturbance and to seed dispersal because of transportation of goods.” Uwimana et al. did warn, however, that “the occurrence of 7% crop-wild hybrids among natural L. serriola populations is relatively high [for a predominantly self-pollinating species] and reveals a potential [for] transgene movement from crop to wild relatives [in] self-pollinating crops.”

Lettuce, domesticated about six thousand years ago in a region referred to as the Fertile Crescent, bears little resemblance to its wild ancestors. Hundreds of years of cultivation and artificial selection eliminated spines from the leaves, reduced the latex content and bitter flavor, shortened stem internodes for a more compact, leafy plant, and increased seed size, among several other things. The resulting plant even has a different name, Lactuca sativa (in Latin, sativa means cultivated). However, cultivated lettuce remains closely related to its progenitors, with whom it can cross to produce wild-domestic hybrids. For this reason, there is great interest in the wild relatives of lettuce and the beneficial traits they offer.

Crop wild relatives are a hot topic these days. That’s because feeding a growing population in an increasingly globalized world with the threat of climate change looming requires creative strategies. Utilizing wild relatives of crops in breeding programs is a potential way to improve yields and address issues like pests and diseases, drought, and climate change. While this isn’t necessarily a new strategy, it is increasingly important as the loss of biodiversity around the globe threatens many crop wild relatives. Securing them now is imperative.

There are about 100 species in the genus Lactuca. Most of them are found in Asia and Africa, with the greatest diversity distributed across Southwest Asia and the Mediterranean Basin. The genus consists of annual, biennial, and perennial species, a few of which are shrubs or vines. Prickly lettuce (L. serriola), willowleaf lettuce (L. saligna), and bitter lettuce (L. virosa) are weedy species with a wide distribution outside of their native range. Prickly lettuce is particularly common in North America, occurring in the diverse habitats of urban areas, natural areas, and agricultural fields. It is also the species considered to be the main ancestor of today’s cultivated lettuce.

In a paper published in European Journal of Plant Pathology in 2014. Lebeda et al. discuss using wild relatives in lettuce breeding and list some of the known cultivars derived from crosses with wild species. They write that in the last thirty years, “significant progress has been made in germplasm enhancement and the introduction of novel traits in cultivated lettuce.” Traditionally, Lactuca serriola has been the primary source for novel traits, but breeders are increasingly looking to other species of wild lettuce.

Resistance to disease is one of the main aims of lettuce breeders. Resistance genes can be found among populations of cultivated lettuce, but as “extensive screening” for such genes leads to “diminishing returns in terms of new resistance,” breeders look to wild lettuce species as “sources of new beneficial alleles.” The problem is that there are large gaps in our knowledge when it comes to wild lettuce species and their interactions with pests and pathogens. Finding the genes we are looking for will require “screening large collections of well defined wild Lactuca germplasm.” But first we must develop such collections.

In a separate paper (published in Euphytica in 2009), Lebeda et al. discuss just how large the gaps in our understanding of the genus Lactuca are. Beginning with our present collections they found “serious taxonomic discrepancies” as well as significant redundancy and unnecessary duplicates in and among gene banks. They also pointed out that “over 90% of wild collections are represented by only three species” [the three weedy species named above], and they urged gene banks to “rapidly [acquire] lettuce progenitors and wild relatives from the probable center of origin of lettuce and from those areas with the highest genetic diversity of Lactuca species” as their potential for improving cultivated lettuce is too important to neglect.

Lactuca is a highly variable genus; species can differ substantially in their growth and phenology from individual to individual. Lebeda et al. write, “developmental stages of plants, as influenced through selective processes under the eco-geographic conditions where they evolved, can persist when plants are cultivated under common environmental conditions and may be fixed genetically.” For this reason it is important to collect numerous individuals of each species from across their entire range in order to obtain the broadest possible suite of traits to select from.

One such trait is root development and the related ability to access water and nutrients and tolerate drought. Through selection, cultivated lettuce has become a very shallow-rooted plant, reliant on regular irrigation and fertilizer applications. In an issue of Theoretical and Applied Genetics published in 2000, Johnson et al. demonstrate the potential that Lactuca serriola, with its deep taproot and ability to tolerate drought, has for developing lettuce cultivars that are more drought tolerant and more efficient at using soil nutrients.

Clearly we have long way to go in developing improved lettuce cultivars using wild relatives, but the potential is there. As Lebeda et al. write in the European Journal of Plant Pathology, “Lettuce is one of the main horticultural crops where a strategy of wild related germplasm exploitation and utilization in breeding programs is most commonly used with very high practical impact.”

Coming Up in Part Two: Can cultivated lettuce cross with wild lettuce to create super weeds?